There has been much concern in the prior art with the external surfaces of medically implantable prostheses, including breast implants. One such concern with these implantable prostheses is the natural tendency for the human body to surround an implanted foreign substance with fibrous tissue to thereby isolate them from surrounding normal tissues. This encapsulation, called capsular contracture when involving mammary prostheses, is not desirable as there is a tendency for the scar tissue or capsule to contract. Ultimately, the capsule has been found to contract to assume a nearly spherical shape thereby detracting from the desired aesthetic appearance of a human breast. Furthermore, this can cause discomfort and may require correction through a surgical procedure which is not desired.
In the prior art, some attention has been paid to this problem and one of the approaches to partially solving this problem has been to texture the external surface of these implants. Various kinds of texturing have been tried in the prior art including open cell structure texturing as disclosed in U.S. Pat. Nos. 4,889,744 and 5,007,929, the disclosures of which are incorporated herein by reference. As described therein, a method may be used as a silicone elastomer surface is formed to create an open cell structure in the external surface thereof. This method generally comprises the steps of partially curing the external surface, applying a layer of solid particles to the surface before the layer is fully cured, fully curing the layer, and then dissolving the solid particles from the surface by dipping the surface into a solvent. The solvent must be chosen so as to dissolve only the solid particles and not affect the silicone elastomer surface. One example given is crystalline sodium chloride (salt) for the particles and water as the solvent. As disclosed therein, the method requires fully curing the layer with the solid particles being intact within the layer prior to dipping the layer into the solvent. However, the inventors believe that, in actual practice, the implant is first scrubbed with a stiff brush to perforate the encapsulated salt to speed up the dissolving process.
This prior art method is successful in forming a textured surface, but the texturing is limited by the nature of the method itself. First of all, the solid particles must be on the edge of the surface, or so closely adjacent as to be touching particles which are on the edge of the surface so that the solvent may reach the particles and dissolve them. As can be appreciated, water has no affect upon an elastomer so that any particles which are not so oriented will not be reached by the water as the water does not penetrate the silicone elastomeric material. Thus, the texturing is shallow and does not penetrate to any appreciable extent the surface of the layer. Furthermore, there is no interaction between the particles and the silicone elastomer as the particles are dissolved by the solvent. This is because the silicone elastomer has been fixed in shape in the curing step and the step of dissolving the particles takes place after the curing step. Again, this tends to limit the "roughness" of the surface whose shape is determined as the elastomer is cured and prior to the dissolving step. Thus, while there is some texturing achieved with the prior art method, it is limited in depth and "roughness".
In order to improve upon this prior art method of texturing silicone elastomeric surfaces which cover medically implantable prostheses and other devices, the inventors herein have succeeded in developing a method which not only increases the depth of the texturing and the "roughness" of the texturing, it also eliminates a separate dissolving step for the solid particles. Thus, not only is the surface better "texturized", so that it should experience greater physiological compatibility by optimizing tissue ingrowth with blood vessel proliferation (vascularity), this improved method may be performed faster and at less expense in manufacturing. Briefly, the method of the present invention includes the step of first forming a silicone elastomer laminate base and then adding a layer of silicone elastomer by dipping the mandrel and laminate base layer into a silicone rubber dispersion, thereby creating a silicone elastomer top layer with a tacky surface, applying a coating of volatilizable particles to the tacky surface, and then finish curing the surface by heating it at an elevated temperature for a prescribed time period which also volatilizes the particles. Thus, the particles decompose and volatilize into constituent gases which interact with the layer of silicone elastomer as it is being cured to dramatically increase the "roughing" of the surface. The volatilizing during the curing step can be likened to a "boiling" action which is much more active than merely dissolving the particles in a solvent as in the prior art. Furthermore, particles which are completely covered with the silicone elastomer layer will still volatilize and force their way to the surface. Thus, the depth of the texturing achieved can be much greater than in the prior art method.
While the principal advantages and features of the present invention have been described above, a more complete and thorough understanding of the invention may be attained by referring to the drawings and description of the preferred embodiment which follow.